Bug 1517909 - Format build telemetry time correctly when seconds has no fractional component. r=nalexander a=NPOTB
It turns out that Python's `datetime.isoformat` method will leave off the
fractional component of seconds if it would be all zeroes, but the voluptuous
`Datetime` validator wants it to be present, so it's possible to hit an error
if you run mach at exactly an integer second.
This patch switches from `isoformat` to `strftime` with an explicit format
string instead.
Differential Revision: https://phabricator.services.mozilla.com/D15981